1. Field of the Invention
The present invention relates to a push-pull circuit connected to an inductive load, and to a DC/DC converter, a solar charging system, and a movable body provided with the push-pull circuit.
2. Description of the Related Art
A push-pull circuit for converting DC voltage outputted from a DC power source into a pulse voltage is sometimes used in a power source device. For example, the power source device disclosed in Japanese Laid-open Patent Application No. 2000-50402 comprises a push-pull circuit 101, a transformer 102, a full-bridge circuit 103, and a step-up chopper circuit provided between a capacitor 104 and a main battery 105, as shown in FIG. 19.
The power source device disclosed in Japanese Laid-open Patent Application No. 2000-50402 cuts off a relay contact 106 when the main battery 105 is charged from an auxiliary equipment battery 100, and performs a step-up operation via the push-pull circuit 101, the transformer 102, the full-bridge circuit 103 (used as a rectifier circuit), and the step-up chopper circuit. Also, the power source device disclosed in Japanese Laid-open Patent Application No. 2000-50402 allows conduction through the relay contact 106 when the auxiliary equipment battery 100 is charged from the main battery 105, and performs a step-down operation via the full-bridge circuit 103, the transformer 102, and the push-pull circuit 101.
The power supply device disclosed in Japanese Laid-open Patent Application No. 2000-50402 is limited to application in a hybrid electric automobile. Accordingly, in the power supply device disclosed in Japanese Laid-open Patent Application No. 2000-50402, the main application is to charge the auxiliary equipment battery 100 from the main battery 105, and charging the main battery 105 from the auxiliary equipment battery 100 is mainly carried out for reverse transmission of electricity from the auxiliary equipment battery 100 to an engine startup motor when there is insufficient power in the main battery 105. Therefore, the power supply device disclosed in Japanese Laid-open Patent Application No. 2000-50402 does not require high power transmission.
However, there is a need for higher-efficiency power transmission in accompaniment with higher capacity cells mounted in automobiles in recent years. A particular problem in electric automobiles is cruising distance, and a system that makes efficient use of electricity is essential.
Nevertheless, in the power supply device disclosed in Japanese Laid-open Patent Application No. 2000-50402, there is a problem in that power transmission between the main battery 105 and the auxiliary equipment battery 100 cannot be carried out efficiently.
In the case that a large current flows to the push-pull circuit 101 during a step-up operation, a very large surge voltage due to parasitic inductance (not shown in FIG. 19) of the transformer 102 is generated when a transistor Q1 or Q2 switches from an on state to an off state. This surge voltage is substantially proportional to the value of the electric current that flows to the transistor immediately prior to switching. Therefore, the surge voltage increases in correspondence to a higher electric current flowing to the circuit, and the circuit is more readily destroyed. When Zener diodes ZD1, ZD2 are provided in the manner shown in FIG. 20 in order to prevent the circuit from being destroyed by the surge voltage, there is a large loss that occurs when the surge component equal to or greater than the Zener voltage (the shaded portion of FIG. 21) is discarded to GND via the Zener diode ZD1 in the case that the surge voltage is generated at a connection point A between the drain of the transistor TL1, which is an N-channel metal oxide semiconductor field effect transistor (MOSFET), and one end of the low-voltage windings of the transformer TR1, and when the surge component equal to or greater than the Zener voltage (the shaded portion of FIG. 21) is discharged to GND via the Zener diode ZD2 in the case that he surge voltage is generated at a connection point B between the drain of the transistor TL2, which is an N-channel MOSFET, and one end of the low-voltage windings of the transformer TR1.
The battery B1 of FIG. 20 corresponds to the auxiliary equipment battery 100 of FIG. 19. The transistor TL1, which is the N-channel MOSFET of FIG. 20, corresponds to the transistor Q1, which is an NPN bipolar transistor, in FIG. 19, and the transistor TL2, which is the N-channel MOSFET of FIG. 20, corresponds to the transistor Q2, which is an NPN bipolar transistor, in FIG. 19. The transformer TR1 of FIG. 20 corresponds to the transformer 102 in FIG. 19. The transistors TH1 to TH4, which are N-channel MOSFETs in FIG. 20, correspond to the transistors Q3 to Q6, which are N-channel MOSFETs, in FIG. 19. The capacitor CH of FIG. 20 corresponds to the capacitor 104 in FIG. 19. In FIG. 20, the parasitic inductance of the low-voltage windings of the transformer TR1 is shown as parasitic inductors PL1 and PL2.